It is known that in chairs, especially those for offices, or more generally for places of work, in order to cushion the backrest with respect to the seat, cushioning devices are used provided with one or more springs. In the simplest devices a single torsion spring is normally used, mounted coaxial to a pin which rotatably connects a metal upright of the backrest with a metal base of the seat. To be more exact, such torsion springs have one end attached to the upright of the backrest and another end attached to the base of the seat.
Such known cushioning devices have the disadvantage, however, that they do not support a large number of stresses, or actuation cycles, or bending of the backrest with respect to the seat. Torsion springs, in fact, do not exceed the stresses required by safety regulations, such as the European norm EN 1335-3, which lays down that devices mounted on chairs must resist up to at least 120,000 cycles before breaking.
It is clear therefore that these known devices do not meet the required characteristics, and thus make the chairs on which they are applied not very reliable, unsafe and poor quality.
From U.S. Pat. No. 4,756,575 it is known a frame assembly for a chair which includes a base member, a seat support member and a backrest member. The front end of the seat support member is pivotally mounted on the front end of the base member and a seat suspension spring is provided for the purposes of urging the seat support member to its elevated position, while allowing the inclination both of the seat and of the backrest. The seat suspension spring is a compression spring mounted rearward of and extending longitudinally of the backrest member. In particular the compression spring is compressed between an end of the base member and an external wing of the backrest member. Because the compression spring is disposed completely outside with respect to the backrest member, the same compression spring is not only totally visible, with negative impact to the aesthetics of the chair, but also subject to knocks and damages.
One purpose of the present invention is to achieve a cushioning device for a backrest of a chair that will resist a very high number of actuation cycles, more than the 120,000 laid down by the legislation in force.
Another purpose of the present invention is to achieve a cushioning device which is simple and economical to make, and which can be replaced or repaired easily in the event of malfunctions or breakage, without necessarily requiring the replacement of the whole device or of the chair.
In accordance with these purposes, the Applicant has devised, tested and embodied the present invention, to overcome the shortcomings of the state of the art, and to obtain other purposes and advantages.